According to Dittmar (U.S. Pat. No. 1,018,746) phosphoric acid is obtained in a high degree of purity by mixing a phosphate of an alkali metal or of an alkaline earth metal with hydrochloric acid or hydrofluoric acid in excess xe2x80x9cas the chlorid or the fluorid of the metal is insoluble in hydrochloric acid or hydrofluoric acid respectively, whereupon the metal is precipitated as an insoluble chlorid or fluorid, the phosphoric acid being liberated and remaining in solution so that it can be separated by filtration.xe2x80x9d In the case of phosphates or superphosphates of alkaline earths, for example calcium or magnesium phosphate, excess of hydrofluoric acid is used.
Hechenbleikner (U.S. Pat. No. 1,313,379) treats phosphate rock with a mixture of dilute hydrofluosilicic and hydrofluoric acids.
Cross (U.S. Pat. No. 2,493,915) treats phosphate rock with a minimum amount of moisture present with sulfuric acid, hydrogen chloride or even hydrogen fluoride or any other strong acid to form phosphoric acid together with the calcium salt of the acid used in treatment. This phosphoric acid is then extracted with sulfuric ether, amyl alcohol or other volatile solvent. Any organic solvent which will dissolve the phosphoric acid but will not dissolve the calcium salt may be used. Accordingly, his claims are directed to reacting calcium phosphate material in a tower by introducing into the tower and passing upwardly through the calcium phosphate material therein a gaseous reagent selected from the group consisting of hydrogen chloride and hydrogen fluoride.
Lapple (U.S. Pat. No. 3,323,864) points out that a xe2x80x9chydrofluoric acid-based process is generally impractical for commercial operation because the rate of rock attack with an acid of practical strength is too slow and because of the high cost of this acid.xe2x80x9d
Lynn (U.S. Pat. No. 3,792,153) notes, with regard to the digestion of phosphate rock to form phosphoric acid, that current xe2x80x9cprocesses digest the mineral with sulfuric acid during which process HF may be liberated, creating a severe pollution problem for the phosphate industry. By-product gypsum is formed in quantities much too large for the available market, and the digestion is relatively slow because of the formation of the insoluble gypsum.xe2x80x9d
Russian Patent No. 880,974 refers to decomposition of phosphate rock with perchloric and hydrofluoric acid and then with nitric acid.
Claim 2 of Belgian Patent No. 750,498 relates to heating a mixture of calcium phosphate mineral with a water-soluble fluoride and acid, wherein the acid can be, inter alia, hydrofluoric acid.
Aqueous hydrofluoric acid is mixed with phosphate rock in a relatively small reactor or in one or more small vessels for a retention time of at least 10 seconds, usually between 10 and 30 minutes. After complete reaction, a slurry of calcium fluoride (CaF2), phosphoric acid and some excess HF is separated, using normal separation techniques, such as a basic table filter. The first wash from the filter, containing from 10 to 15% P2O5 and 2 to 10% HF, is recycled back to the reaction section to regulate the amount of solids in the slurry to the filter and to recover the P2O5 that is washed from the filter cake. The return stream, which contains some dissolved Ca++ ions, must be added to the reactor slurry after the reaction between the phosphate and the HF has taken place to prevent the formation of very small crystals of CaF2.
Filter grade acid of from 10 to 33% (usually from about 25 to 28%) P2O5 is concentrated from 40% to  greater than 60% P2O5 in a standard vacuum evaporator. The product concentration step is similar to that used in conventional wet-process phosphoric acid production. However, in the subject process essentially no solids are precipitated during this step, thus reducing a major waste material-handling problem that is currently faced by the industry.
Clarification techniques and/or activated carbon absorption may be used to produce a clear-product acid for use as technical or food-grade acid.
The CaF2 filter cake (is in the form of pseudomorphs having greater than 95% CaF2) recovered from the filter is mixed with sulfuric acid and thermally treated in a rotary kiln in which HF fumes are liberated from the solids. The fumes generated during this acidification/thermal treatment are scrubbed from the exit gas stream using standard absorber technology. A by-product of the rotary (regeneration) kiln is calcium sulfate which is stacked in contained piles. The CaF2/H2SO4 reaction, thermal treatment, and HF recovery steps of the process rely on proven existing technology and process equipment commonly used in the HF production industry.
The scrubbed/recovered HF, less than 50% and normally less than 37% HF, is subsequently concentrated in an HF concentrator and recycled to the reactor; excess HF is marketed. Gas vented from the HF concentrator is returned to the HF recovery system.
Scrubbing HF vapors from the reactor, from the filter and from the H3PO4 concentrators serves to both control and recover the HF emissions from these pieces of apparatus. Recovery of the HF and return to the reaction system are advantageous from both an environmental and economic point of view. Clean vapors from the HF process scrubber(s) and from the HF recovery absorber(s) are vented to the atmosphere.
An advantage of this invention is that it requires significantly fewer pieces of process equipment that are generally smaller in size than the equipment used in the conventional phosphoric acid-producing methods. A further advantage is a reduction in reactor retention time from the 6 to 12 hours necessary to effect good rock dissolution and to grow desired-size gypsum dihydrate (CaSO4.2H2O) crystals (essential for good filtration and high P2O5 recovery) to only about 10 minutes to effect good conversion of the phosphate rock to CaF2. [Crystals of CaF2 take the form of the phosphate rock (pseudomorphs) in the feed, and dissolution and recrystallization are not required.]
A still further advantage is the reduction in agitation requirements for reaction; the conventional process requires agitation to effect complete dissolution of phosphate rock. The degree of agitation is extremely high and results in high-energy usage and in erosion of the equipment located inside the reactor. The agitator drives generally require up to or greater than 100 HP for each compartment (up to 13); the subject process requires only one low energy agitator per reaction vessel which operates at a more gentle level of agitation, just enough to keep the solid particles suspended without causing particle attrition. (If a pipe reactor is used, no agitation is required.)
Another advantage is in the required management of heat, which is essential to the proper operation of the conventional process, in which heat is normally removed using a large flash cooler that operates at about 4 inches of Hg absolute pressure. The vacuum is generated by the use of a steam ejector followed by a barometric condenser or by a vacuum pump. The subject process requires a substantially lower level of removal of the heat of reaction. This is the result of the formation of pseudomorphs rather than the growing of gypsum crystals. It is anticipated that no heat removal will be required.
An additional advantage is in process control and automation. In the conventional process, the free sulfate level in the reactor is probably the most important process variable and is also one of the most difficult to measure and control on a steady/predictable basis. The automation of the measurement and/or control of the free sulfate level has been attempted many times over the years, but the problem has remained essentially unsolved. In addition, when an adjustment is made to the system, it takes several hours to see the final results. This is due in part to the long retention time in the reaction section and to slow changes that occur with the growth of CaSO4 crystals. Total automation of the conventional system is very difficult. As the major process controls for the subject invention are based primarily on chemical analysis and feed rates of the raw material inputs to the reactor, both process control and automation are easily handled. Instead of a free sulfate measurement, a free HF level is monitored. The free HF level is not nearly as critical as is the free sulfate level. The major emphasis is to maintain a sufficiently high free HF level in the reactor loop so that all of the rock can be converted to the CaF2 pseudomorphs. There is a quick response time to process changes because of the short retention times required and because there is no crystal growth rate to consider.
A further advantage involves the filtration/separation area. In the conventional process a problem arises with scaling inside the main valve, pans and drain lines. Solids (mainly calcium sulfate.xH2O) precipitate out of the supersaturated acid during filtration and form hard scale deposits, which have to be removed physically, such as manually chipping them out, and this results in unplanned operating down time. The-solids build-up also results in excess wear on rotating surfaces and/or surfaces of the filter. The instantly-claimed process utilizes the same basic primary filter (table, tilting pan, or belt). However, the acid is not supersaturated with CaF2, and scaling of the filter does not occur.
The on-stream time for the filter (and plant as a whole) is thus longer.
A still further advantage involves storage at all acid concentrations. Management of solids that accumulate in phosphoric acid after it is filtered is currently a major problem within the industry. This problem occurs throughout the time the acid is filtered, stored, concentrated, shipped, and used. A high level of dissolved solids in merchant grade acid (a common occurrence) makes it very difficult for an end user (such as a fluid fertilizer dealer) to use the product. This means that the dealer will generally have to use a more expensive superphosphoric acid for clear liquids or use a solid monoammonium phosphate (MAP) (contains solids from the acid) to produce a suspension-based fertilizer. Bulk shipments of merchant grade acid post precipitates vast amounts of solids creating high P2O5 losses and clean-up problems. The acid produced by the HF process does not create these problems.
Filter-grade acid (about 26 to 29% P2O5) from conventional processes is supersaturated with gypsum. After the acid is separated using a filter, normal practice lets the acid sit in a storage tank where fine gypsum crystals precipitate. This results in a sludge material that is comprised of very small crystals, and which is generally recycled from the bottom of the tank(s) back to the primary filter and/or reactor system. This in turn tends to reduce the overall filtration rate.
The filter-grade acid that has settled is then fed to an evaporation system where it is concentrated to a merchant-grade acid (about 54% P2O5). This is typically done in two steps; the first step goes to about 40% P2O5, and the second, to the final 54% level. Solids precipitation is a major problem in both of these steps.
In the instantly-claimed process there is very little or no solids accumulation. The reaction depends upon an in-situ reaction where the CaF2 is formed as a pseudomorph having the same shape and size as that of the phosphate rock feed. Essentially no dissolution and/or re-precipitation takes place. In addition, impurities contained in the phosphate rock do not dissolve into the acid, but remain inside the pseudomorphs, and this results in a very clean acid, having low dissolved solids and low impurities.
One of the major advantages of the instantly-claimed process is its ability to produce a clean acid, one clean enough to be easily made into an 18-46-0 grade diammonium phosphate (DAP) without adding urea or ammonium nitrate. Also, merchant-grade acid from the instant process can be readily shipped in rail cars and stored at dealer sites without solids accumulation. This makes it possible for fluid fertilizer dealers to use a less expensive raw material than they are currently using. The presently-disclosed process also makes it possible for major phosphate fertilizer companies that manufacture superphosphoric acid (SPA, xcx9c70% P2O5) to produce their product easier than is possible using the conventional sulfate-based process because of the lack of solids precipitation and lower acid viscosity. This allows the production of an alternative product with lower P2O5 values with the same benefits as SPA.
Another advantage involves concentration/evaporation. The concentration of acid from the filter is necessary to yield a product that can be further used to ship or to produce a variety of products (including solid and liquid fertilizers). The concentration step is carried out in steam-heated vacuum evaporators. In the conventional process the filter-grade acid is generally between 26 and 29% P2O5. The evaporation step is fairly straight-forward with the filter-grade acid being concentrated in two steps. The first step is from 26% to 40% P2O5. The obtained product is stored before it is concentrated to 54% P2O5. The storage at 40% is for two purposes: the first is to let some of the solids settle, and the second is that 40% P2O5 is a strength traditionally used to produce DAP.
Prior to, during, and following the concentration step, there are major problems with solids and impurities. These solids tend to fill the storage tanks and scale the processing equipment; dissolved impurities result in a higher -boiling point for the acid (higher steam requirement). The solids also cause some erosion in the circulation and heating systems of the evaporators, restriction of flow in the heat exchangers, and reduced heat transfer in the heat exchangers.
The instantly-claimed process produces filter-grade, intermediate-grade, or merchant-grade acids with very few solids present. This is because there is essentially no CaF2 dissolved in the acid, and essentially all of the metal impurities are trapped inside the CaF2 filter cake. Therefore, there are no scaling, plugging, or erosion problems associated with the process.
As the filter-grade acid from the subject process is purer than that produced by conventional methods, the resulting boiling point is lower. This makes it easier to concentrate material and results in significant savings in the amount of steam used for heating. The improved flow through the heat exchangers and lower scaling result in higher heat transfer which results in higher steam efficiencies.
A major advantage of the instantly-disclosed process is its ability to use a wide variety of phosphate rock supplies. In addition, the HF can be recycled using proven technology and this eliminates the need for an outside source of HF (an important economic advantage for the process).
Generally speaking, the best grade of phosphate rock in the United States has been consumed steadily over the last century. Remaining deposits show increased levels of impurities, such as magnesium, iron, and aluminum. These higher levels of impurities make the production of phosphoric acid and the phosphate-based fertilizers more difficult and more expensive. Some current mining areas in Florida are being skipped and/or covered over and lost because of high magnesium levels in the phosphate rock. There are numerous marginal-grade rock deposits in developing countries that are currently considered too small and too expensive to develop and/or process. The presently-disclosed process will allow economical use of such rocks. This, in turn, will be a major boost to the agricultural economy and development of these countries.
The presently-claimed primary process can use almost any phosphate rock source while immobilizing impurities inside individual filter-cake particles. This is done by the formation of pseudomorphs, which tend to let the HF penetrate into the rock particle and then let the pure phosphoric acid exit out of the particle. The outer ring of CaF2 on the pseudomorph appears to act like a microfilter, and the impurities remain trapped inside the particle as metal fluorides. In the conventional sulfuric acid based processes, the phosphate rock is totally dissolved, and the impurities are free to react and to go into solution and/or to precipitate.
The instantly-disclosed process significantly increases the overall usable phosphate reserves for the United States and for the world. The combination of smaller processing equipment/overall facility size and the ability of the subject process to use impure rock deposits found in developing countries will potentially allow these countries to become more self-sufficient in agriculture.
Although there are many further advantages, the principal advantages are:
1) The majority of involved metallic materials (iron, aluminum, magnesium, etc.) stay with the CaF2, yielding a significantly cleaner product phosphoric acid, making direct production of technical and/or food-grade acid possible.
2) The cleaner acid obtained makes it possible for the fertilizer industry to achieve and maintain an 18-46-0 fertilizer grade for diammonium phosphate (DAP) without addition of urea or ammonium nitrate. The absence of heavy metals, such as cadmium and lead, in the product will significantly increase the international market potential for some of the DAP currently produced in the U.S.
3) The clean acid is also useful for producing a premium fluid fertilizer with or without polyphosphates.
4) The world""s supply of available/usable phosphate rock is expanded significantly because the process makes it economical to produce phosphoric acid from rock having higher levels of impurities.
5) The filter cake by-product from the process is primarily calcium fluoride, which can be converted to CaSO4, using proven existing technology. This in turn releases HF which is recycled to the reaction section of the process; excess HF can be sold as a marketable commodity. If the CaF2 produced is not converted, it can be sold as a marketable raw material for the HF production industry.
6) Little or no process cooling is required for the process, thus significantly reducing the size and concentration of fluorine laden cooling ponds. A calcium hydroxide final scrubber will essentially remove/recover all fluorine air emissions and the solids from the scrubber.
7) CaF2 would be regenerated to CaSO4 and HF for further reaction or for sales rather than being placed in cooling ponds.
8) The subject process has significantly lower retention times in the reaction section. Smaller and simpler process equipment is thus required.
9) Energy usage is significantly reduced due to lower agitation and pumping requirements, as well as lower heat requirements during evaporation.